N'Zoth is the most powerful Old God still corpreal.*
N'Zoth was the primary agent behind Neltharion's corruption.#
N'Zoth maintains the Rift of Aln in the city of Ny'Alotha.*
Ny'Alotha is inhabited by the Naga.
N'Zoth was the primary agent behind the transformation of the Highborne.
Ny'Alotha was one of the five citadels of the Old Gods before their destruction by the Titans.*
The Emerald Dream is being invaded THROUGH the Rift of Aln, in the city of Ny'Alotha.

Too many forces have been linked over the past three expansions to be ignored. Too much content that would so easily work together.
We WILL see Queen Azshara, N'Zoth, the Emerald Dream and Ny'Alotha in the future.

That's really, really interesting. I never thought of the fate of Y'Shaarj equaling the stuff mentioned in the Halls of Stone. It definitely would make sense: the Titans killed Y'Shaarj, and he almost destroyed the continent as a result. And even then, he wasn't destroyed completely (as we now know). Definitely not a worthwhile endeavor to repeat with the others. I had always imagined that by "destroying Azeroth" they meant in a big dramatic sort of way, where the entire planet physically fell apart.

But having the Curse of Y'Shaarj x 5, possibly in different and even more destructive varieties, could definitely qualify as a planet-killer.

Blizzard has implied that it may have been C'Thun and Yogg-Saron's defeat that allowed for the Catacylsm to take place at all - their deaths may have somehow weakened the planet enough for Deathwing to pull off what he did. And as you mentioned in the OP, there was hints on the PTR of C'Thun appearing somehow in Silithus and seriously messing the place up. So yeah, they're bad news even after they're dead.

My personal, all-jokes-aside theory on the matter is that perhaps N'Zoth is like Tzeentch from Warhammer 40k. In a way, it would be as you said: Deathwing's defeat played into his hands. But I have a feeling it may have done so in ways we can't yet imagine. Tzeentch is notorious for being a "God of Assholes" in that he often manipulates others, even his own, because of some grand scheme that no one else can comprehend dictates it.

That'd be cool - N'Zoth not being the most physically powerful, but the smartest, the most insidious. Him being the master schemer of the Old Gods would be an interesting take.